This invention relates to a method and apparatus for providing additional or replacement anodes to prevent corrosion of a steel platform positioned at an offshore location.
Present day offshore platforms used in the oil and gas industry are often formed of large-diameter pipe elements in the form of three or more vertical or slanting legs interconnected or reinforced by cross-bracing tubular members. Such bottom-supported platforms have been used in waters up to 1025 feet deep. The deepwater platforms may have more legs which may be tapered. For example, one deepwater platform off the California coast has eight legs that are made of 72 inch diameter pipe at the ocean floor and taper upwardly to 48 inch diameter pipe at sea level. Cross-bracing members are mostly 36 or 42 inches in diameter. In addition, the platform is provided with sixty 24 inch diameter vertical pipes, risers or well conductors which are grouped near the center of the platform and through which individual wells are drilled. Further, the platform supports vertical pipe risers through which oil and gas may be separately pumped down to an ocean floor pipeline and thence to shore.
In order to protect the present offshore platforms from corrosion in sea water, the structural members of the platform are provided with a cathodic protection system which comprises fixedly securing to a plurality of the structural members a number of sacrificial anodes which are preferably made of aluminum, zinc, or an alloy of these and/or other metals, in a manner well known to the art. Anodes are often used which are made of magnesium which gives out a larger current than aluminum alloy anodes, although having a shorter life, as disclosed in U.S. Pat. No. 2,571,062. In addition to anodes being fixedly mounted on a platform, they may be suspended therefrom by chains or cables as shown in U.S. Pat. Nos. 2,870,079; 4,089,767; 4,292,149; and 4,056,446.
Corrosion in sea water is an electrochemical process. During the chemical reaction of metals with the environment to form corrosion products (such as rust on steel), metallic atoms give up one or more electrons to become positively charged ions, and oxygen and water combine with the electrons to form negatively charged ions. The reactions occur at rates which result in no charge build-up. All the electrons given up by metal atoms must be consumed by another reaction.
The cathodic protection of offshore platforms and other structures exposed to marine environments is an art which has been practiced for many years. The objective of all anode systems is to provide current flow from anodes to a platform so as to elevate the polarization level of the platform within the "protected" range; that is, the level at which electron emission from the protected platform to the surrounding sea water is substantially inhibited, thereby suppressing corrosion of the platform.
Cathodic protection is a process which prevents the anodic corrosion reaction by creating an electric field at the surface of the metal so that current flows into the metal. This prevents the formation of metal ions by setting up a potential gradient at the surface which opposes the electric current which arises from the flow of electrically charged ions away from the surface as the product of corrosion. The electric field must be of adequate strength to ensure that metal ions are fully prevented from escaping.
A source of the electric field which opposes the corrosion reaction may be a current supplied from the preferential corrosion of a metal anode with different electrochemical properties in the environment, and which has a stronger anodic reaction with the environment than does the offshore structure. Thus, current flows to the structure from the additional anode, which itself progressively corrodes in preference to the structures. This technique is known as sacrificial anode cathodic protection. This method is used extensively for the protection of offshore platforms, drilling rigs, submarine pipelines, etc.
When sacrificial system is chosen, the weight of material required to provide the protection current for the protected lifetime of the structure is calculated from a knowledge of the current demand and also the specific electrochemical properties of the anode alloys.
The calculated weight of anode alloy cannot be installed all in one piece but must be distributed over the structure in the form of smaller anodes to ensure uniform distribution of current. In order to select the best size and shape of anode, the total current demand of the structure both at the beginning and end of its life must be considered. The anode must deliver adquate current to polarize the structure and build up cathodic chalks, but also must be capable of delivering the required mean current for the structure when 90% consumed.
Thus, on most offshore platforms a multiplicity of anodes are arranged on the various structural members of the platform. These anodes are generally attached to the platform before the platform is lowered to the ocean floor. Generally, the well conductor pipes are not provided with anodes as the conductors are lowered through the deck and driven into the ocean floor after the platform is in position. It has been found that by installing numerous anodes on the structureal elements of the platform in the vicinity of the well conductors that the conductors, which are welded at the top to the platform or are in electrical contact with the platform, are adequately protected against electrolytic corrosion in the sea water.
A major problem is encountered with a platform positioned over an offshore oil field with a calculated life of twenty years at the time the field was first put into production. In actuality, the field proved to have a life of forty years or more. Thus, it may be seen that the cathode protection system for the platform is probably inadequate to protect the steel platform from sea water corrosion for this longer period. Hence, it is generally necessary to add additional anodes to the underwater portion of the platform structure. On small simple platforms in shallow water, it is sufficient to lower an anode down through the water on a hoist cable and have a diver connect it to its underwater position on the platform. However, the large deepwater platforms containing a large number of well conductors comprise a maze of vertical and cross-bracing members but oftentimes there are not a sufficient number of members located next to a cluster of well conductors, to which anodes can be secured to provide adequate protection for the well conductors.